This invention relates to an imaging method using nuclear magnetic resonance and more particularly to a method which makes it possible to perform chemical shift imaging at a high speed.
In order to obtain an image of an object with the aid of NMR signals, it is necessary to separate and discriminate the NMR signals from the object according to respective positions of the object. One method for this purpose is to provide position information by applying a gradient magnetic field to the object to provide different static magnetic field intensities in accordance with the positions of the object so as to obtain different resonance frequencies or phase encode amounts at the positions.
The principal theory of this method has been reported in Journal of Magnetic Resonance, Vol. 18 page 69 (1975) and Physics of Medicine and Biology, Vol. 25, page 751 (1980).
One method of imaging an object using NMR signals is chemical shift imaging. The chemical shift is a phenomenon that spins provide their different resonance frequencies according to positions at a molecular structure since those spins even with the same nuclei are influenced by different magnetic fields according to their surrounding molecular structure. The chemical shift is a very important phenomenon since it gives information relative to the molecular structure of an object. Examples of imaging spin density images (hereinafter referred to as chemical shift images) discriminated from one another according to their chemical shift mainly consist of (a) an extension method of a Fourier imaging method reported by Maudsley et al in Journal of Magnetic Resonance, Vol. 51 page 147, 1983 and (b) a method proposed by Dixon in Radiology, Vol. 153, page 189, 1984.
The method of (a) makes it possible to separate and measure the chemical shifts by adding the frequency dimension to the spatial dimension. In this method, an image of an object is divided into L.times.M (L, M: integer) picture elements (pixels) in the case when it is a two dimensional plane, and N (N: integer) signal points are sampled for each pixel. The value of L or M is determined according to the space resolution used. For example, if L=M=128, L.times.M=16,384. Although N signal points can be sampled by one measurement, the time close to a longitudinal relaxation time of an object (about 1 sec in the case of a human body) must elapse before a subsequent measurement. Thus, the measurement time of 4.6 hours is required for L.times.M measurements.
On the other hand, the method of (b) constitutes an image containing only the information relative to a specific chemical shift from the sum of and the difference between two images in the cases of intervals .tau..sub.1 =.tau..sub.2 and intervals .tau..sub.1 .noteq..tau..sub.2 in a pulse sequence of 90.degree.-.tau..sub.1 -180.degree.-.tau..sub.2 -(signal measurement), 90.degree. and 180.degree. represent RF pulses flipping the spin by 90.degree. and 180.degree., respectively. This method is very practical since the time required for the signal measurement is only twice as long as the time required to construct one image. However, the phase change of the spin due to the chemical shift is almost the same as or smaller than the phase change due to the inhomogeneity of the static magnetic field, and both changes cannot be discriminated from each other. Dixon et al proposed to remove the influence due to the static magnetic field inhomogeneity by calculating a square root of the sum of the squares of a real part and an imaginary part after the complex Fourier transform, i.e. absolute value. Nevertheless, both chemical shifts cannot be discriminated depending on the spin numbers corresponding to two chemical shifts. Further, in the method of (b), the chemical shifts to be detected are limited to two kinds. Also, in order to obtain two chemical shift images, the measurement must be carried out under two different conditions.